1. Technical Field of the Invention
The present invention relates generally to a method and system for applying a liquid, which contains a resin or the like dissolved in a solvent, e.g., a polyimide solution, on a substrate, such as a semiconductor wafer or an LCD substrate (a glass substrate for a liquid crystal display), to form a liquid film.
2. Description of Related Art
As one of semiconductor fabricating steps, there is a process for applying a polyimide on a substrate, such as a semiconductor wafer, in order to form a protective film or interlayer insulating film of a semiconductor device. In such a process, it is conventionally required that the thickness of a coating film formed on the surface of the substrate is uniform over the whole. In recent years, with the scale down of circuits and so forth, it is required that the inplane uniformity of the thickness of the film is higher than that of conventional films.
In the circumstance, as one of coating film forming systems, there has been studied a system for further diluting a chemical, which contains a polyimide dissolved in a solvent, to prepare a coating liquid and for rotating a semiconductor wafer (which will be hereinafter referred to as a wafer) as shown in, e.g., FIG. 11, to discharge the coating liquid 12 from a nozzle 11 onto the surface of the wafer W while gradually moving the nozzle 11 in a radial direction from the center of the wafer W, to spirally apply the coating liquid 12 in the same manner as a picture drawn with a single stroke of the brush.
In such a system, the lines of the spirally coating liquid are tightly arranged without providing gaps in radial directions of the wafer W to integrate the lines of the coating liquid, so that a coating film having a high inplane uniformity of thickness is formed. Specifically, for example, coating data including values, such as the rate of discharge of the coating liquid, the speed of rotation of the substrate, and the traveling speed of the nozzle, are prepared on the basis of previously measured data, and the coating liquid is applied on the basis of this data.
However, since the above described system uses a nozzle having a small diameter of, e.g., about 100 μm, cavitation occurs in the passage of the nozzle, and bubbles of the coating liquid generated by the cavitation are broken on the surface of the wafer W, so that there is a problem in that an uncoated region formed by breaking the lines of the coating liquid is generated. For that reason, for example, various measures to provide a degassing mechanism upstream of the nozzle for removing bubbles remaining in the coating liquid have been studied. However, the effects of all of the measures are insufficient.
Using a small-diameter nozzle, the width of the lines of the coating liquid applied on the wafer W is decreased. In order to tightly arrange such thin lines of the coating liquid, the nozzle must be very precisely moved with respect to the wafer W on the basis of the above described coating data. However, although the coating data have a predetermined error range every condition, if all values of the above described three conditions, e.g., the rate of discharge of the coating liquid, the speed of rotation of the substrate and the traveling speed of the nozzle, are values approximating the limit of error, the adjacent lines of the coating liquid are partially spaced from each other, so that there are some cases where a linear uncoated region is generated in each gap.
Moreover, although the coating liquid is applied so as to apparently draw a spiral, the nozzle does not spirally move in fact, and the position of the nozzle is changed by the balance between the rotation of the wafer W and the straight-line motion of the nozzle in radial directions of the wafer W. Therefore, the coating liquid on the wafer W varies slightly step-wise from a microscopic point of view, and if this step increases, a linear uncoated region is generated between the adjacent lines of the coating liquid.